Colloidal semiconductor nanocrystals have generated great fundamental interest in recent years and continue to exhibit tremendous promise for developing advanced materials. (Heath, J. R., Ed. Acc. Chem. Res. 1999; Alivisatos, A. P. Science 1996, 271, 933-937; Brus, L. E. J. Chem. Phys. 1986, 90, 2555) The size-dependent emission is probably the most attractive property of semiconductor nanocrystals. For example, differently sized CdSe nanocrystals can be prepared that emit from blue to red with very pure color. These nanocrystal-based emitters can be used for many purposes, such as light-emitting diodes, (Sundar, V. C.; Lee, J.; Heine, J. R.; Bawendi, M. G.; Jensen, K. F. Adv. Mater. 2000, 12, 1102; Schlamp, M. C.; Peng, X. G.; Alivisatos, A. P. J. Appl. Phys. 1997, 82, 5837-5842) lasers, (Artemyev, M.; Woggon, U.; R., W.; Jaschinski, H.; W., L. Nano Lett. 2001, 1, 309; Klimov, V. I.; Mikhailovsky, A. A.; Xu, S.; Malko, A.; Hollingsworth, J. A.; Leatherdale, C. A.; Eisler, H. J.; Bawendi, M. G. Science 2000, 290, 314-317) biomedical tags, (Han, M.; Gao, X.; Su, J. Z.; Nie, S. Nat. Biotechnol. 2001, 19, 631-635; Bruchez, M.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P. Science 1998, 281, 2013-2016; Chan, W. C. W.; Nie, S. M. Science 1998, 281, 2016-2018) and the like. For this reason, the control of the photoluminescence (PL) properties of semiconductor nanocrystals has been a major goal for developing the synthetic chemistry for colloidal semiconductor nanocrystals. In turn, the lack of viable synthetic methods that permit the desired level of control over the PL properties of semiconductor nanocrystals appears to have hampered progress in this area, and delayed the timely development of advanced applications for these unique materials.
The emission properties of semiconductor nanocrystals can be characterized by four fundamental parameters, namely the brightness, the emission color, the color purity, and the stability of the emission. Due to quantum size effects, (Brus, L. E. J. Chem. Phys. 1986, 90, 2555) the band gap of CdSe nanocrystals increases as their size decreases, and thus the emission color of the band-edge PL of the nanocrystals shifts continuously from red (centered around 650 nm) to blue (centered around 450 nm) as the size of the nanocrystals decreases. Because the emission color of a semiconductor nanocrystal is strongly dependent on its size (Brus, L. E. J. Chem. Phys. 1986, 90, 2555) and shape, (Peng, X. G.; Manna, L.; Yang, W. D.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P. Nature 2000, 404, 59-61) the color purity of the emission becomes dependent on the size and shape distribution of a nanocrystal sample. Therefore, the control of the emission color and color purity is likely a matter of the control of the size and shape, as well as size and shape distribution of the semiconductor nanocrystals, regarding which recent studies have provided a reasonable knowledge basis. (Peng, X. G.; Manna, L.; Yang, W. D.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P. Nature 2000, 404, 59-61; Peng, Z. A.; Peng, X. G. J. Am. Chem. Soc. 2001, 123, 1389-1395; Manna, L.; Scher, E. C.; Alivisatos, A. P. J. Am. Chem. Soc. 2000, 122, 12700-12706; Peng, X. G.; Wickham, J.; Alivisatos, A. P. J. Am. Chem. Soc. 1998, 120, 5343-5344; Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706-8715) The other two emission properties of brightness and emission stability, however, cannot yet be correlated with specific structural parameters or some adjustable nanocrystal growth condition associated with a given synthetic scheme. As a result, the PL brightness, measured by PL quantum yield (QY) or efficiency, and the stability of the emission of the “as-prepared” semiconductor nanocrystals are not easy to predict and generally vary from synthesis to synthesis. The ability to control a synthetic parameter such that PL efficiency (PL QY) of semiconductor nanocrystals could be boosted would be extremely valuable.
CdSe nanocrystals with relatively narrow size distributions (low polydispersity) and relatively high crystallinity became available in the early 1990s by use of dimethylcadmium as the cadmium precursor. (Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706-8715; Brennan, J. G.; Siegrist, T.; Carroll, P. J.; Stuczynski, S. M.; Reynders, P.; Brus, L. E.; Steigerwald, M. L. Chem. Mater. 1990, 2, 403) This organometallic approach has been further developed in terms of the control over the size, (Peng, X. G.; Wickham, J.; Alivisatos, A. P. J. Am. Chem. Soc. 1998, 120, 5343-5344) shape, (Peng, X. G.; Manna, L.; Yang, W. D.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P. Nature 2000, 404, 59-61; Peng, Z. A.; Peng, X. G. J. Am. Chem. Soc. 2001, 123, 1389-1395; Manna, L.; Scher, E. C.; Alivisatos, A. P. J. Am. Chem. Soc. 2000, 122, 12700-12706) and size/shape distribution of the resulting CdSe nanocrystals. More recently, alternative routes to CdSe have been developed using safe, inexpensive, and environmentally sound cadmium precursors and ligands, (Peng, X. Chem. Eur. J. 2002, 8, 334; Peng, Z. A.; Peng, X. J. Am. Chem. Soc. 2001, 123, 183-184; Qu, L.; Peng, Z. A.; Peng, X. Nano Lett. 2001, 1, 333) thereby providing a readily available synthetic method for CdSe nanocrystals. In principle, with knowledge regarding the control over the size/shape and size/shape distribution of CdSe nanocrystals, the emission color and the purity of the color can be controlled to a certain extent. However at present, the purity of the emission color of a CdSe nanocrystal sample is still significantly worse than that of the single particle emission. For example, the typical full width at half maximum (FWHM) of the PL peak of a CdSe nanocrystal ensemble at room temperature, is around 27-40 nm, (Sundar, V. C.; Lee, J.; Heine, J. R.; Bawendi, M. G.; Jensen, K. F. Adv. Mater. 2000, 12, 1102; Talapin, D.; Rogach, A. L.; Kornowski, A.; Haase, M.; Weller, H. Nano Lett. 2001, 1, 207) noticeably broader than that observed by single dot spectroscopy (typically <20 nm). (Heath, J. R., Ed. Acc. Chem. Res. 1999) These data indicate the relatively nonhomogeneous emission properties of the nanocrystals within the sample.
The control over the PL QY and emission stability of the CdSe nanocrystals synthesized through either the traditional organometallic approach or the alternative routes also remains rather poor. For example, the best PL QY reported for the as-prepared nanocrystals at room temperature is around 20% in the wavelength range between 520 and 600 nm, and only a few percent or lower in the wavelength regions above 600 nm and below 520 nm. (Sundar, V. C.; Lee, J.; Heine, J. R.; Bawendi, M. G.; Jensen, K. F. Adv. Mater. 2000, 12, 1102; Qu, L.; Peng, Z. A.; Peng, X. Nano Lett. 2001, 1, 333; Talapin, D.; Rogach, A. L.; Kornowski, A.; Haase, M.; Weller, H. Nano Lett. 2001, 1, 207) Neither the stability nor the reproducibility of the PL QY are predictable using current synthetic methods. (Qu, L.; Peng, Z. A.; Peng, X. Nano Lett. 2001, 1, 333) With some inorganic and organic surface passivation following synthesis, the PL QY of CdSe nanocrystals can be boosted to as high as over 50% in the 520-600 nm window, (Talapin, D.; Rogach, A. L.; Komowski, A.; Haase, M.; Weller, H. Nano Lett. 2001, 1, 207; Peng, X. G.; Schlamp, M. C.; Kadavanich, A. V.; Alivisatos, A. P. J. Am. Chem. Soc. 1997, 119, 7019-7029; Dabbousi, B. O.; Rodriguez-Viejo, J.; Mikulec, F. V.; Heine, J. R.; Mattoussi, H.; Ober, R.; Jensen, K. F.; Bawendi, M. G. J. Phys. Chem. B 1997, 101, 9463-9475; Hines, M. A.; Guyot-Sionnest, P. J. Phys. Chem. 1996, 100, 468) but the efficiency for the orange-red color window is still quite low. This problem is especially severe for red emissions (around 650 nm), where the PL QY of nanocrystals in solution is nearly zero, like the present, as-prepared CdSe nanocrystals. (Sundar, V. C.; Lee, J.; Heine, J. R.; Bawendi, M. G.; Jensen, K. F. Adv. Mater. 2000, 12, 1102; Talapin, D.; Rogach, A. L.; Komowski, A.; Haase, M.; Weller, H. Nano Lett. 2001, 1, 207)
Therefore, what is needed are new methods to prepare semiconductor nanocrystals that afford the ability to boost and control the photoluminescence quantum yield (PL QY) of as-prepared samples, especially for CdSe nanocrystals. If possible, these methods would allow manipulation of the purity of the emission color, by controlling the full width at half maximum (FWHM) of the nanocrystal PL peak. It is also desirable to develop methods that could provide emission peaks sufficiently sharp so as to approach those observed by single dot spectroscopy (in the 20 nm range). Due to the failure of orange-red emitting materials in general, efforts are especially needed for materials that emit in the wavelength region from about 600 nm (orange) to about 650 nm (red). To accomplish these difficult goals of controlling and tuning the photoluminescence (PL) properties of CdSe, as well as other semiconductor nanocrystals, it would also be desirable to obtain detailed information that correlated nanocrystal growth parameters with emission properties. The present invention demonstrates that, despite previous failures to control PL properties, especially the PL QY, of semiconductor nanocrystals, such control is possible, and can be readily correlated with synthesis and crystallization parameters.